I said this elsewhere, and I will say it again: "Ye gods, but I love this forum." Thanks, Solonor, for your post; it certainly pointed me in the right direction.
It's even simpler than how you described. I'm taking this down for myself, and anyone else who might want to try importing a fictional player (I'm not talking historical here) from another universe:
In the parallel universe, go to League Setup, Import/Export, Export Rosters. This creates a .txt roster file in that saved game folder. Open that .txt file in a word processor and edit it, removing all players except the one(s) that you want to "transfer."
Next, and this is key (thanks to you Solonor for mentioning it), you must alter the team ID of the player(s) to some number beyond the number of teams in the main universe. Otherwise, the transferred player will simply join the corresponding team in his new universe. If there is no matching team ID, he ends up as a free agent, as Soloner said.
Something else that's important: His player ID must be in the range of the numbers already established in his new universe. You cannot do as above and make up a player ID, because he never shows up in that case. You have to select a "sacrificial lamb" in the main universe and use his player ID for the player to be transferred. It's best to do this anyway because you if left the player ID as it is and it does match an existing player where he is going, he will overwrite that player. Better to control things by selecting a very insignificant free agent and using his ID rather than having a Hideki Matsui
replace a Derek Jeter!
Once the roster text file has been edited, save it and return to OOTP. Open the game that contains the main universe and import the saved roster file. If all went well, you should see the transferred player(s) in the free agent pool.
I did a test as commissioner in edit mode and found that all of bio stuff and all of the offensive and pitching ratings were the same after transfer. Interestingly, whatever player characteristics that are not contained in the roster file were subject to change. For example, the transferred player took on the personality of the replaced player (how bizarre). These are editable in commissioner mode, however. And, of course, the transferred player has no service time or contract data, which is appropriate.
He also has no career statistics, which I think is also appropriate, given that he has changed leagues. One other thing to note: His player history will be that of the replaced player, but player history can be edited as established in this thread:
Player Histories
By the way, I am fully aware that you could replicate this the long way with the Create Fictional Player button and editing the new player profile (you cannot, however, just edit that "sacrificial lamb" that I spoke of, because I see no way to delete his past statistics). My way may sound longer, but I think creating and editing a new player to "become" the transferred player would actually be more work. And you would still want to edit his history anyway, making some mention of his past in the parallel universe. Besides, the player export/import transferral idea appeals to me for some reason.
Well, thanks again, Solonor, for the tip. Abu Taha, this does nothing for you, I'm afraid, because you need the league and team structure in place and matching the data in the roster file. If that data matched your current universe, all you would be doing is overwriting your current players. If it does not match, then I am guessing nothing would happen, or you would get all your players dumped into the free agent pool. I did not mean to hijack your thread, and I hope you get a positive answer to your question, but I am afraid Charlie is right about importing leagues. Good luck.